Reefs impacted by category five cyclone Yasi in 2011 are demonstrating their capacity to bounce back, with new research showing an average 4.4 per cent coral cover increase over two years.

The finding follows the largest assessment ever conducted on the Great Barrier Reef after a cyclone or major disturbance.

Lead researcher and acting director of tourism and stewardship at the Great Barrier Reef Marine Park Authority, Roger Beeden, said the results show the ecosystem retains its natural ability to recover.

“The Reef is constantly changing, and for many thousands of years it has undergone cycles of disturbance and regeneration because of cyclones,” said Dr Beeden. 

“What we’re now witnessing in many areas between Cairns and Bowen, where cyclone Yasi swept through, is good regrowth of surviving corals and large numbers of new coral colonies.

“As we expected, the results varied among individual reefs, with some showing a coral cover increase of more than nine per cent, while others experienced a decline of nearly six per cent.

“Without the current crown-of-thorns starfish outbreak in the far northern part of the Marine Park and other pressures, it’s likely that an even greater overall average increase in coral cover on reefs hit by cyclone Yasi would have occurred.”

The severe tropical cyclone affected about 600 kilometres of the 2300 kilometre long Reef, mainly between Cairns and Townsville.

A series of 840 reef health and impact surveys was conducted in the wake of the storm, followed by recovery surveys two years later on a subset of reefs located 150 kilometres north and south of the cyclone track.

Dr Beeden said the study sought to demonstrate the relative ferocity and scale of cyclone Yasi.

“Between 2004 and 2015, nine cyclones greater than a category three crossed the Marine Park, including the recent cyclones Marcia and Nathan. And yet between 1970 and 2003, there were no cyclones higher than a category three,” he said.

“The potential for a cyclone to cause structural damage to reefs is driven by three factors — intensity, the radius of the storm, and the duration of extreme conditions near reefs. 

In this instance, cyclone Yasi packed a punch on all three.

“It’s been by far the largest severe cyclone we’ve had in recent years, with wind gusts of up to 295 kilometres per hour and gale force winds that extended more than 600 kilometres.

“When it made landfall it was the most powerful cyclone to have crossed the Queensland coast since 1918.

“In other words, this was an extraordinary cyclone that will leave a legacy for many years to come in some parts of the Marine Park.”

Dr Beeden said Marine Park visitors could help the process of coral recovery by reducing pressures on the Reef.

“The average increase in coral cover is encouraging and shows the Reef retains the natural processes that drive and aid recovery, such as connectivity between reefs, and the ability for corals to repopulate and regrow,” he said.

“However, these processes need to be fostered through our own actions. Every bit counts, whether it be reducing marine debris, being careful not to anchor on coral, or making sure you don’t fish in a green zone.

“The results also highlight the importance of a control program that’s culling the crown-of-thorns starfish, and the efforts of thousands of farmers and graziers in reducing sediment and nutrient run-off from their

properties.”

About 15 per cent of the total coral reef area within the Marine Park is estimated to have sustained some level of damage from cyclone Yasi, with approximately four per cent sustaining some structural damage.

Geological records show evidence of repeated damage on the Great Barrier Reef from cyclones over the past 5000 years.

The paper ‘Impacts and recovery from severe tropical cyclone Yasi on the Great Barrier Reef’ is published in the scientific journal PLOS One.